Successful amplification of SINEs by the LINE machinery requires their transcription to be initiated by internal promoters. Typical SINEs are derived from cellular genes transcribed by RNA polymerase III, containing internal promoters. For example, Ll-dependent Alu elements are derived from 7SL RNA genes encoding the RNA scaffold of the signal recognition particle, whereas MIRs are derived from tRNA genes. The 5' parts of MIRs are homologous to tRNA genes, and the 3' terminal portions are homologous to the respective LINE elements on which their proliferation depends (Fig. 1A). MIR shares its 3' terminus with L2 elements, MIR3 with L3 elements (6,8,9). MIR and MIR3 SINEs, like their LINE2 and LINE3 counterparts, are very old and their contribution to genomic rearrangements in the human genome seems to be minimal because no homologous recombination between ancient SINEs associated with human disease has been reported.
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